1. Field Of The Invention
This invention relates to memory arrays and, more particularly, to circuits for generating reference values for use with the sensing circuits of a memory array.
2. History Of The Prior Art
Integrated circuit memories utilize sensing circuits to extract data from the memory cells of the array. Sensing circuits generate a reference voltage or current to compare with a level stored in the memory cells. The result of the comparison is used to generate a logic "0" or "1" at the output of the sensing circuit. The number of sensing circuits contained in an integrated circuit memory is basically equal to the number of bits that the device furnishes as a parallel output. For example, a byte-wide memory requires eight sets of sensing circuits and a word-wide memory requires sixteen sets of sensing circuits.
The most accurate method of generating a reference voltage or current for non-volatile memories is to construct a reference cell which uses devices that exactly match the memory cells. These reference cells will then track the characteristics of the memory array in spite of variations in processing, temperature, and supply voltage. This results in very reliable, high performance sensing operations. Since the reference cells have been required for each set of sensing circuits, the number of these cells and their support circuitry can use significant die area. Moreover, the devices of the reference cells are often adjusted or trimmed during production testing for improved performance. This trimming of a large number of reference cells can consume a significant amount of test time, substantially increasing the cost of the memory array.
An approach to reducing the number of reference cells is to "mirror" the current from a single reference cell to each set of sensing circuits throughout the die, thus reducing die cost, by minimizing die area and test time. This concept is relatively straight forward and has been used in prior art devices. However, this approach is limited from a performance standpoint due to the inherent inaccuracy of the mirroring circuits and the relatively slow power-up time when a large number of sensing circuits are connected.
It is, therefore, desirable to reduce the number of components necessary to provide reference values in sensing circuitry for integrated circuit memory arrays while maintaining the speed of operation and the accuracy of the reference values produced.